Structural fit-up, tack welding and mechanical assembly before final weld-out.
Fit-up is where individual pieces become one assembly. The shop tack welds parts into position for structural fit-up, builds sub-assemblies, and handles mechanical assembly and bolt-up work before final welding or shipping. Field-installation preparation is part of the same step, since an assembly that will be bolted together on site needs to fit the first time it goes up. Getting fit-up right before final welding also keeps distortion down, which matters on anything headed to inspection.
